1. Field of the Invention
This invention relates to network protector relays used to control circuit breakers and, more particularly, to such network protector relays for circuit breakers connecting feeders to low-voltage secondary power distribution networks. The invention also relates to a method of controlling a circuit breaker employing two trip characteristics.
2. Background Information
Low-voltage secondary power distribution networks consist of interlaced loops or grids supplied by two or more sources of power, in order that the loss of any one source will not result in an interruption of power. Such networks provide the highest possible level of reliability with conventional power distribution and are, normally, used to supply high-density load areas, such as a section of a city, a large building or an industrial site. Each power source is a medium voltage feeder supplying the network and consists of a switch, a transformer and a network protector. The network protector includes a circuit breaker and a control relay. The control relay senses the transformer voltages, the network voltages and the line currents, and executes algorithms to initiate breaker tripping or reclosing action. Trip determination is based on detecting reverse power flow, that is, power flow from the network to the primary feeder.
Examples of network protector relays are disclosed in U.S. Pat. Nos. 3,947,728; 5,822,165; and 5,844,781.
Traditionally, network protector relays were electromechanical devices, which tripped the circuit breaker open upon detection of power flow in the reverse direction. The electromechanical network protector relays are being replaced. One type of electronic network protector relay mimics the action of the electromechanical relay by calculating power flow.
Another type of electronic network protector relay uses sequence voltages and currents to determine the direction of current flow for making tripping decisions. Sequence analysis, upon which such relays are based, generates three vector sets to represent a three-phase voltage or current: (1) a positive sequence vector, (2) a negative sequence vector, and (3) a zero sequence vector. U.S. Pat. No. 3,947,728 discloses a sequence based network protector relay, which uses the positive sequence current and positive sequence voltage vectors to make trip decisions.
More recently, digital sequence based network protector relays have been utilized which periodically sample (e.g., 8, 16, 32 times per cycle) the current and voltages.
FIG. 1 illustrates a secondary power distribution network system 1, which includes a low-voltage grid 3 servicing various loads 5. The secondary network bus or grid 3 is energized by multiple sources in the form of feeders 7a, 7b, 7c, 7d. Feeders 7a and 7b are supplied directly from substations 9a and 9b, respectively. Each of the feeders 7a-7d respectively includes a feeder bus 11a-11d, a switch 13a-13d, a feeder transformer 15a-15d, and a network protector 17a-17d. The secondary network system 1 and its components are three-phase wye or delta connected, although FIG. 1 shows these as a single line for clarity. Each of the network protectors 17a-17d includes network protector circuit breakers 19a-19d and network protector control relays 21a-21d, respectively.
As disclosed in U.S. Pat. No. 5,822,165, which is incorporated by reference herein, the control relays 21a-21d each include a microcontroller-based circuit (not shown) which monitors the network phase to neutral voltages Vn (e.g., Van, Vbn, Vcn), the transformer phase to neutral voltages Vt (e.g., Vat, Vbt, Vct), and the feeder currents I (e.g., Ia, Ib, Ic).
Typically, control relays include a communication module for communication with a remote station over a communication network (or xe2x80x9ccommunication subsystemxe2x80x9d in order to avoid confusion with the secondary network bus 3). For example, one or more MPCV control relays, which are marketed by Cutler-Hammer of Pittsburgh, Pa., may be connected to the communication subsystem (e.g., without limitation, INCOM physical communication layer, and PowerNet or IMPACC Series III communication software, as marketed by Cutler-Hammer) to allow remote access to protector measurement data of interest. In turn, the control relays perform breaker trip and reclose functions.
Advances in solid-state technology continue to improve the functionality of network protector relays.
U.S. Pat. No. 5,822,165 discloses a network protector relay, for example, whereby the flexibility of more powerful processing resources relative to first generation solid-state relays and the even older electromechanical designs allow for providing a more robust and safe low-voltage power distribution network.
The primary responsibility of a network protector relay is to recognize and react to backfeed conditions (i.e., power leaving a low-voltage network grid). In the event that the amount of backfeeding power meets the programmed setpoints of the relay, then the relay trips the network protector circuit breaker and, thus, isolates the feeder circuit. The other major responsibility of the relay is, of course, deciding when the transformer voltage conditions are within programmed parameters relative to the network bus voltages, in order to command a reclosure of the network breaker.
Referring to FIG. 2, a phasor diagram 31 shows a traditional network relay trip characteristic. The network voltage phasor reference is shown as vector VN 33 at 0xc2x0. A normal, lagging network load current vector is included for reference as vector ILOAD 35. A network relay should trip the protector circuit breaker on backfeed conditions that will occur when the feeder circuit is faulted or when the feeder circuit is opened. ISC is shown representing a feeder fault backfeed vector 37, lagging the 180xc2x0 reference 39 due to the dominating network transformer leakage inductance and feeder cable inductance combination. For the case of an open feeder, the dominating term is typically the transformer secondary winding magnetizing inductance, as indicated by current vector IM 41.
FIG. 2 shows the network protector relay tripping characteristic region 43 (shown in cross-hatch in FIG. 2) with a +5xc2x0 counterclockwise tilt 45, the purpose of which is to trip on backfeeding currents that may be highly leading the 180xc2x0 reference 39 due to system cable charging currents indicated by IC, which is represented by vector 46. A corresponding non-trip region 47 is shown above the trip region 43. The threshold line of the trip region 43 is sloped 5 degrees to compensate for phase shift (e.g., in the network transformers; in the current transformers which measure the currents). This avoids unnecessary tripping in response to temporary reverse current conditions which could be caused for instance by a regenerative load on the network 3.
The described vectors 33,35,37,41,46 are not drawn to a particular scale, but are simply graphical representations of various system conditions. RT represents the reverse trip setpoint 48 of the network protector relay. In this regard, the circuit breaker 103 has a rated value of current. Typically, the reverse trip setpoint RT 48 is about 0.2% of the rated value of current and is associated with the positive sequence current vector ISC 37. The intention is that at every network protector relay location, each relay has all possible system current backfeed conditions fall within the trip region 43 of FIG. 2.
There is room for improvement in network protector relays.
These needs and others are satisfied by the invention, which is directed to the trip functionality of a network protector relay, which employs two different trip characteristics by adding a xe2x80x9cGull-Wingxe2x80x9d trip region to the existing traditional trip region.
As one aspect of the invention, a network protector relay for controlling a circuit breaker connected between a polyphase feeder bus and a polyphase network bus comprises: means for sampling polyphase current flowing through the circuit breaker and polyphase network voltage on the network bus to generate digital polyphase current samples and digital polyphase network voltage samples; and digital processor means comprising: means for generating a positive sequence current vector and a positive sequence voltage vector from the digital polyphase current samples and the digital polyphase network voltage samples, means for tripping the circuit breaker open in response to the positive sequence current vector being in a first trip region of a first trip characteristic with respect to the positive sequence voltage vector, the first trip characteristic defined by a first reverse trip setpoint and a first positive angle, and means for tripping the circuit breaker open in response to the positive sequence current vector being in a second trip region of a second trip characteristic with respect to the positive sequence voltage vector, the second trip characteristic defined by a second reverse trip setpoint and a second negative angle.
The first positive angle may be about +5 degrees. Preferably, the second negative angle is about xe2x88x925 degrees.
The digital processor means may include means for storing the second negative angle as a predetermined negative angle, or means for configuring the second negative angle.
Preferably, the circuit breaker has a rated value of current between the polyphase network bus and the polyphase feeder bus associated with the positive sequence current vector, and the reverse trip setpoint is about 0.2% of the rated value of current.
As another aspect of the invention, a method of controlling a circuit breaker connected between a polyphase feeder bus and a polyphase network bus comprises: sampling polyphase current flowing through the circuit breaker and polyphase network voltage on the network bus to generate polyphase current samples and polyphase network voltage samples; generating a positive sequence current vector and a positive sequence voltage vector from the polyphase current samples and the polyphase network voltage samples; employing a first reverse trip setpoint and a first positive angle to define a first trip characteristic having a first trip region with respect to the positive sequence voltage vector; employing a second reverse trip setpoint and a second negative angle to define a second trip characteristic having a second trip region with respect to the positive sequence voltage vector; tripping the circuit breaker open in response to the positive sequence current vector being in the first trip region of the first trip characteristic; and alternatively tripping the circuit breaker open in response to the positive sequence current vector being in the second trip region of the second trip characteristic.